It has been said that more than 32% of the mercury emitted in the United States to the atmosphere is from coal-burning utilities. Should further mercury control emissions from municipal solid waste and medical waste incinerators be mandated, the percentage of mercury released to the atmosphere from coal-burning utilities would greatly increase. A low concentration of mercury, on the order of 1 part per billion by volume (ppbv), is found in flue gas when coal is burned. The primary forms of mercury in the flue gas are elemental mercury and oxidized mercury (believed to be mercuric chloride).
U.S. Pat. No. 6,521,021 represents an example of the Thief Process for removal of mercury from flue gas. The Thief Process is a cost-effective variation to activated carbon injection (ACI) for removal of mercury from flue gas. In this scheme, partially combusted coal from the furnace of a pulverized coal power generation plant is extracted by a lance and then re-injected into the ductwork downstream of the air preheater. Recent results on a 500-lb/hr pilot-scale combustion facility show similar removals of mercury for both the Thief Process and ACI.
Another process is represented by U.S. Pat. No. 6,447,740 which is directed to a process for chlorine oxidizing gaseous pollutants in the flue gas stream water to soluble chlorides and then using a water scrubbing process. In particular, chlorine is injected to oxidize pollutants of the flue gas stream and then in oxidized form they are more readily available for removal.
Many of the above described processes have been in place in mercury removal processing but with the Mar. 15, 2005 EPA issued final Clean Air Mercury Rule, there is a continuing need for further developments of cheap and effective ways of eliminating mercury from flue gas. Especially since worldwide it has been estimated that two-thirds of the mercury air emissions are anthropogenic (human caused emissions). As previously stated, there are a number of currently available control technologies that coal-fired power plants can use to reduce their emissions of mercury to the atmosphere. For example, controls for sulfur dioxide, oxides of nitrogen and small particles that have already been installed remove some of the mercury before it is released from the stack. The effectiveness of these technologies for mercury removal varies, depending on characteristics of the coal and the configuration of the power plant. In some cases a plant might consider changing the type of coal that it burns in order to get better mercury control from its existing control devices.
Control technologies specifically used to reduce mercury emissions from coal fired power plants have recently begun to be used on some power plants with success. The most highly advanced technology, activated carbon injection has been used on facilities that burn municipal solid waste for the past decade. Particles of activated carbon are injected into the exit gas flow, downstream of the boiler. The mercury attaches to the carbon particles and is removed in a traditional particle control device. Several other control technologies to reduce mercury emissions from coal-fired power plants are being developed and tested, such as those of the above-mentioned patents, but have not yet been deployed at the commercial scale.
To date, the processes developed have been very expensive and complex. For example, U.S. Pat. No. 6,447,740 reports that the costs of capturing mercury can be as high as $100,000/lb.
It can be seen that there is a need for continuing efforts, especially since recent nutritional emphasis on fish eating as important to a healthy diet have become part of the lowering of cholesterol campaign. Fish are important in a healthy diet. They are a lean, low-calorie source of protein. However, some fish may contain methylmercury or other harmful chemicals at sufficiently high levels to be a concern. Federal, state and local governments issue fish consumption advisories when the fish are unsafe to eat. The advisories may suggest that people avoid eating certain kinds or certain amounts of fish. Some advisories apply to specific water types (like lakes). Some may focus on groups of particularly sensitive people. Some advisories include notices of “no restriction” to tell us that certain fish are safe to eat. As states increase the waters they monitor for contaminated fish, both the number of advisories and the waters where it is safe to eat fish are increasing.
Elimination of flue gas
In short, there is a government mandated incentive to develop less costly, more effective methods of removing mercury from flue gas, especially from coal fired power plants, but also from medical incinerators and the like.
The primary objective of the present invention is to develop a new cost effective, easy to install flue gas mercury removal system which in nearly all instances, removes 70% of the mercury and in most instances 90-95% of the mercury.
The method of accomplishing the above primary objective will be apparent from the following detailed description of the invention.